1. Field of the Invention
The present invention relates to an electro-hydrostatic actuator (EHA) in which a motor, a pump, and an actuator are integrated, to a drive device used for the same, and to a control method used for the same. In more detail, the present invention relates to an electro hydrostatic actuator excellent in a snubbing function of relaxing impact at a retracted position and an extended position at the time of retracting and extending the landing gear of an airplane, and a drive device used for the same, and a control method used for the same.
2. Description of the Related Art
Conventionally, an airplane is equipped with a hydrostatic supply system, and the landing gear is retracted or extended by an actuator with utilizing hydrostatic force thereof. In reference to the actuator for retracting and extending the gear, for relaxing impact due to collision between a piston end and a cylinder end at a retracted position or an extended position, a snubbing mechanism for reducing travelling speed of the piston is provided.
In some of the above snubbing mechanism, when the piston comes close to a cylinder bottom or a gland nut, a running amount of fluid into and from the cylinder is decreased by narrowing down fluid path or by other way, so that the travelling speed of the piston is reduced.
In this case, since the running amount of the fluid is temporarily decreased, the fluid flowing into the actuator gets highly pressurized, so that breakage failure etc. of related equipments may be caused. In order to prevent such breakage failure of equipments, a relief valve or the like is arranged in a hydrostatic circuit, so that the pressure of the fluid flowing into the actuator is maintained to not more than a predetermined value.
For example, Japanese Patent Application Publication No. 1996-61310 offers a proposal relating to a hydrostatic cylinder used for airplane landing gear or the like. Specifically, a snubbing piston capable of moving a preset stroke from a cylinder bottom is arranged so as to form a buffering fluid chamber, and a predetermined amount of fluid is introduced into the buffering fluid chamber via an orifice at the time of extending a piston. When the piston is abutted with the snubbing piston at the time of retraction, a predetermined amount of the fluid is discharged through the orifice, so that a running amount of the fluid into and from a cylinder is decreased, thus controlling and reducing travelling speed of the piston. Such a hydrostatic cylinder is proposed.